1. Field of the Invention
The present invention relates to a transmission power control device, and more particularly to a downlink transmission power control device for a mobile communication system using a spread spectrum communication method.
2. Description of the Related Art
A lot of attention is currently being paid to the Code Division Multiple Access (hereinafter referred to as CDMA) method as a multiple access (plural connection) method that enables a plurality of users to communicate at the same time by sharing wireless transmission paths. The CDMA method uses spread spectrum techniques and divides the same frequency band among a plurality of users. In order to increase the subscriber capacity in the CDMA method, it is essential to control transmission power with high precision.
In the CDMA method, the conventional method of controlling downlink transmission power is as described in the following. A desired signal-Interference Power Ratio (hereinafter referred to as SIR) for a receive signal is previously set as a target SIR in a mobile station. A small value of SIR means that the ratio of interference power to the power of a signal itself, is small, while a large value of SIR means that the ratio of interference power to the power of a signal itself is large.
The mobile station monitors the SIR of a received signal and compares a monitored SIR with a target SIR. When the monitored SIR is smaller than the target SIR, the mobile station transmits a transmission power control command to the base station instructing increase of the transmission power. When the monitored SIR is larger than the target SIR, the mobile station transmits a transmission power control command to the base station instructing decrease of the transmission power. The base station modifies the transmission power of a transmission signal to a corresponding mobile stations according to this transmission power control command.
However, if the number of mobile stations associated with a single base station is increased, the SIR of the received signal of each mobile station deteriorates. If the SIR of a monitored receive signal is deteriorated, each mobile station will transmit a transmission power control command to the base station instructing increase of transmission power, based on the target SIR of each mobile station. The base station will then increase the transmission power of the transmission signals for each mobile station. For each mobile station, increase in transmission power of transmission signals to other mobile stations causes further deterioration in the SIR. A mobile station with deteriorated received signal SIR then transmits a transmission power control command to the base station instructing increase of transmission power, based on the target SIR of each mobile station. If this vicious circle is repeated, a point is reached where the transmission power can no longer be increased, and communication quality is degraded.
An object of the present invention is to provide a transmission power control device that solves the above mentioned problems.
The present invention provides a transmission power control device that achieves the above described object, comprising: a total transmission power monitoring section, for monitoring a first total output power being transmitted by a base station to a plurality of mobile stations, and outputting a monitor signal; a target total transmission power setting section, for calculating a second total transmission power value appropriate for the base station and outputting the power value as a setting signal; and a reference update amount calculating section, for calculating a reference update amount from a difference between the monitored signal and the setting signal, and outputting the reference update amount, wherein an instruction for an update target value of a signal-interference power ratio is sent to the plurality of mobile stations based on the reference update amount.